We have applied the technique of in situ hybridization with probes specific for mRNA encoding the Alpha, Beta, and Gamma chains of the T cell antigen-receptor, interleukin 2 and the interleukin 2 receptor, to explore early molecular events in fetal thymocyte development. Our results show that the Gamma chain locus turns on prior to the Beta chain locus, that day 13-15 thymocytes make IL-2 and that differentiation in vitro of day 14 fetal thymocytes is associated with expression of Alpha chain mRNA and the Lyt2 cell surface molecule, without L3T4 expression.